Axial flow turbine

ABSTRACT

In the axial turbine according to the present invention, a nozzle blade  1  and/or a movable blade  5  has a profile in which a throat-pitch ratio “s/t” is maximized at a blade-central portion in height, wherein “s” being a shortest distance between a rear edge of a nozzle blade (movable blade) and a back side of another nozzle blade that is adjacent to the nozzle blade, and “t” being a pitch of the nozzle blades disposed in the row, minimized in a position between the blade-central portion in height and a blade-root portion and increased from a minimized value to the blade-root portion. This structure enables to provide the axial turbine, which permits to control flow distribution of the working fluid in the height direction of the blade in the passage between the blades of a turbine nozzle unit and a turbine movable nozzle and reduce the blade profile loss and the secondary flow loss at the blade-root portion, thus making a further improvement in the turbine stage efficiency.

TECHNICAL FIELD

The present invention relates to an axial turbine, especially to such anaxial turbine, which has turbine stages formed by combining turbinenozzle units and turbine movable blade units together and permits toimprove remarkably pressure efficiency of the turbine stages.

BACKGROUND TECHNOLOGY

In an axial turbine of a steam turbine or a gas turbine applied, forexample, to a power plant, there have recently been reviewed improvementin thermal efficiency, and especially, improvement in a turbine internalefficiency, by which an economic operation can be carried outeffectively.

A subject to suppress the secondary flow loss due to the secondary flowof working fluid such as working steam or working gas in a turbinenozzle unit or a turbine movable blade unit, of losses including a bladeprofile loss occurring in a turbine blade and the secondary flow loss(secondary loss) of the working fluid, as low as possible, in order toimprove remarkably the turbine internal efficiency, has been addressedas one of significant subjects of study.

FIG. 10 is a view illustrating a structure of a turbine nozzle unitcalled the “straight blade”, which is conventionally applied to theaxial turbine. A plurality of nozzle blades 1 (so called the “stationaryblades”) is placed in a row in a circumferential direction of a turbineaxis, not shown, of an annular passage 4, which is formed between anouter diaphragm ring 2 and an inner diaphragm ring 3.

A plurality of turbine movable blades 5 is placed in the circumferentialdirection on the downstream side of the nozzle blades 1, so as tocorrespond to the row arrangement of the nozzle blades 1, as shown inFIG. 8. The turbine movable blades 5 are implanted in a rotor disc 6 inthe peripheral direction thereof and are provided at the respectiveouter peripheral ends with a shroud 7, which prevents the working steamor the working gas (hereinafter referred to as the “working fluid mainstream” or merely to as the “main stream”) from leaking.

Detailed description will be given below of a mechanism of occurrence ofthe secondary flow of the working fluid on the nozzle blade 1(hereinafter referred merely to as the “secondary flow”) in the axialturbine having the above-described structure, with reference to FIG. 10,which is a perspective view, in which the turbine nozzle unit is viewedfrom the outlet side of the nozzle blade 1.

The working fluid main stream flows the passage between the blades in acurved shape. At this stage, a centrifugal force is generated from theback (dorsal) side “B” of the nozzle blade 1 toward the front (ventral)side “F”. The centrifugal force is balanced with static pressure so thatthe static pressure on the front side “F” becomes higher.

On the contrary, the flow velocity of the main stream is high on theback side “B”, resulting in the lower static pressure. This causes apressure gradient to occur from the front side “F” towards the back side“B” in the passage between the blades. The pressure gradient also occursin a boundary zone formed on the peripheral wall surface of the outerdiaphragm ring 2 and the inner diaphragm ring 3 in the similar manner.

However, the flow velocity is low and the centrifugal force becomessmall in the boundary zone in the passage between the blades, with theresult that endurance against the pressure gradient from the front side“F” towards the back side “B” cannot be maintained, thus producing thesecondary flow 8 of the working fluid, which is directed from the frontside “F” toward the back side “B”.

The secondary flow 8 collides with the back side “B” of the nozzle blade1 to rise up, thus producing the secondary flow vortexes 9 a, 9 b inconnection portions at which the nozzle blade 1 is connected to theouter diaphragm ring 2 and the inner diaphragm ring 3 so as to supportthe nozzle blade 1.

The energy possessed by the main stream of the working fluid is lostpartially under the influence of development and diffusion of thesecondary flow vortexes 9 a, 9 b, and the wall friction due to thesecondary flow, in this manner, thus becoming a factor responsible forthe remarkably deteriorated turbine internal efficiency. The secondaryflow loss also occurs in the turbine movable blade unit in the samemanner as the turbine nozzle unit.

There have been disclosed many results of research and many proposals toreduce the secondary flow loss due to the secondary flow vortexes 9 a, 9b, which are generated in the passage between the blades.

There has been disclosed for example a turbine nozzle unit, which has aprofile in which a throat-pitch ratio “s/t” expressed by a throat “s”,which is defined by the shortest distance between the rear edge of anozzle blade 1 and the back side “B” of another nozzle blade 1 that isadjacent to the above-mentioned nozzle blade 1, and a pitch “t” of theblades 1 aligned annularly, is maximized at a blade-central portion inheight, on the one hand, and decreased at the blade-root portion and theblade-tip portion, on the other hand, as shown in FIG. 9 (see JapaneseLaid-Open Patent Publication No. HEI 6-272504).

The above-mentioned turbine nozzle unit has advantages as describedbelow in comparison with a turbine nozzle unit or turbine movable bladeunit, which has conventionally been applied for example to a steamturbine and called the “straight blade” type (i.e., the blades placedalong the radial lines, which pass through the center of the turbineaxis and straightly extend radially). In the turbine nozzle unit calledthe “straight blade” type, the loss at the blade-central portion inheight is small, on the one hand, and the loss at the blade-root portionand the blade-tip portion becomes relatively large, on the other hand,as shown in FIG. 5A. Furthermore, in the turbine movable blade unitcalled the “straight blade” type, the loss at the blade-central portionin height is small, on the one hand, and the loss at the blade-rootportion and the blade-tip portion becomes relatively large, on the otherhand, as shown in FIG. 5B. The “loss” means loss of the secondary flowof the working fluid in the following description, unless a definitionis specifically given.

On the contrary, in the turbine nozzle unit having the profile in whichthe throat-pitch ratio “s/t” is maximized at the blade-central portionin height, on the one hand, and decreased at the blade-root portion andthe blade-tip portion, on the other hand, as shown in a dotted line inFIG. 4A, the flow rate of the main stream is decreased at the blade-rootportion and the blade-tip portion in which the larger loss occurs, onthe one hand, and increased at the blade-central portion in height inwhich the smaller loss occurs, on the other hand. Accordingly, the lossgenerated in the whole passage in the turbine nozzle unit becomessmaller in comparison with the turbine nozzle unit called the “straightblade” type.

Furthermore, in the turbine movable blade unit having the profile inwhich the throat-pitch ratio “s/t” is maximized at the blade-centralportion in height, on the one hand, and decreased at the blade-rootportion and the blade-tip portion, on the other hand, as shown in adotted line in FIG. 4B, the loss generated in the whole passage in theturbine movable blade unit becomes smaller in comparison with theturbine movable blade unit called the “straight blade” type, in the samemanner as the above-described turbine nozzle unit.

In addition, with respect to the other results of research, there hasbeen disclosed a turbine nozzle unit called “compound lean” type inwhich the nozzle blades 1 bend relative to the radial lines, which passthrough the center of the turbine axis (which is indicated by thereference sign “E” in FIG. 10) (see Japanese Laid-Open PatentPublication No. HEI 1-106903).

The turbine nozzle unit called the “compound lean” type has a structureas shown in FIG. 7A in which the rear edge of the blade projects in acurved profile from the blade-tip portion and the blade-root portiontowards the blade-central portion in height so as to generate pressingforces, which are applied from the blade-tip portion and the blade-rootportion to the outer and inner diaphragm rings 2 and 3, respectively.Accordingly, the turbine nozzle unit called the “compound lean” typemakes it possible to keep the small pressure gradient in the boundaryzone generated in each of the outer diaphragm ring 2 and the innerdiaphragm ring 3.

The turbine movable blade unit also has a structure as shown in FIG. 7Bin which the rear edge of the blade projects in a curved profile fromthe blade-tip portion and the blade-root portion towards theblade-central portion in height so as to generate pressing forces, whichare applied from the blade-tip portion and the blade-root portion to ashroud 7 and a rotor disc 6, respectively, in the same manner as theabove-described turbine nozzle unit, thus making it possible to keep thesmall pressure gradient in the boundary zone generated in each of theshroud 7 and the rotor disc 6 (see Japanese Laid-Open Patent PublicationNo. HEI 3-189303).

The turbine nozzle unit and the turbine movable blade units, which arecalled the “compound lean” type, have the profile by which the pressingforce applied from the blade-tip portion to the outer diaphragm ring 2as well as the pressing force applied from the blade-root portion to theinner diaphragm ring 3 are given, and the pressure gradient in theboundary zone generated in each of the outer diaphragm ring 2 and theinner diaphragm ring 3 is kept small, thus leading to a larger flowingamount of the main stream.

However, the connection portion of the blade-tip portion to the outerdiaphragm 2 and the connection portion of the blade-root portion to theinner diaphragm 3 originally exist as zones where the secondary flowloss of the working fluid is large. Accordingly, there is a limitationfor further improvement in performance, even when a larger amount of themain stream of the working fluid is supplied to flow.

In view of this fact, the turbine nozzle unit and the turbine movableblade unit, in which the throat-pitch ratio “s/t” is increased at theblade-central portion in height to ensure a larger area of the passage,cause the main stream to flow in a larger amount in a zone at theblade-central portion in height, in which the small loss occurs. It istherefore conceivable that such a structure can make furtherimprovements in performance, thus providing advantages (see JapaneseLaid-Open Patent Publication No. HEI 8-109803).

However, in the turbine nozzle unit and the turbine movable blade unithaving the above-described profile, the throat-pitch ratio “s/t” issmall at both of the blade-root portion and the blade-tip portion, ageometrical discharge angle “α=sin⁻¹(s/t)”, which is calculated from thethroat-pitch ratio “s/t” is also small, and a turning angle becomeslarge.

It is known that, when the turbine nozzle unit and the turbine movableblade unit of the axial turbine generally have the small geometricaldischarge angle or the large turning angle, the boundary zone developson the surface of the blade, thus increasing the blade profile loss.

When the flowing direction of the main stream is drastically changed inthe passage between the blades, the pressure gradient from the frontside “F” towards the back side “B” in the passage between the bladesbecomes large and the secondary flow 8 also becomes large.

In addition, fluid having a low energy, in the boundary zones on thesurface of the blade, which develop in the vicinity of the blade-rootportion and the blade-tip portion, as well as fluid having a low energy,in the boundary zones formed on the peripheral wall surfaces in thepassage between the blades flow together with the secondary flow 8, thusconstituting a factor responsible for the remarkably increased secondaryflow loss.

Especially, the small throat-pitch ratio “s/t” in the blade-root portionmakes the annular pitch “t” small, thus leading to a small throat “s”.The small throat “s” causes a ratio “te/s” of the thickness “te” of therear edge in the throat “s” to become large, since it is required thatthe thickness “te” of the rear edge in the throat “s” has apredetermined value based on the structural requirement of the blade. Asa result, the blade profile loss rapidly increases as shown in FIG. 11.

The turbine nozzle unit and the turbine movable blade unit in which thethroat-pitch ratio “s/t” is increased at the blade-central portion inheight, as well as the other turbine nozzle unit and the other turbinemovable blade unit, which are called the “compound lean” type, any oneof which have been disclosed as one of the results of the recentresearch, have merits and demerits as described above. It is thereforeconceivable that combination of them only in their structure providingthe merits, i.e., realization of a so-called “hybrid blade” makescontribution to the further improvement in the turbine stage efficiency.

An object of the present invention, which was made in view of theabove-mentioned problems, is therefore to provide an axial turbine,which permits to control flow distribution of the main stream in theheight direction of the blade in the passage between the blades of aturbine nozzle unit and a turbine movable nozzle and reduce the bladeprofile loss and the secondary flow loss at the blade-root portion, thusmaking a further improvement in the turbine stage efficiency.

DISCLOSURE OF THE INVENTION

In order to attain the above-described object, an axial turbineaccording to the present invention comprises: a plurality of turbinestages disposed in an axial direction of a turbine shaft, each of theplurality of turbine stages comprising a turbine nozzle unit havingnozzle blades, which are disposed in a row in a circumferentialdirection of an annular passage formed between an outer diaphragm ringand an inner diaphragm ring; and a turbine movable blade unit, which isdisposed on a downstream side of the turbine nozzle unit and has movableblades implanted in a row on the turbine shaft in a circumferentialdirection thereof, wherein the nozzle blades have a profile in which athroat-pitch ratio “s/t” is maximized at a blade-central portion inheight, wherein “s” being a shortest distance between a rear edge of anozzle blade and a back side of another nozzle blade that is adjacent tothe nozzle blade, and “t” being a pitch of the nozzle blades disposed inthe row, minimized in a position between the blade-central portion inheight and a blade-root portion, and increased from a minimized value tothe blade-root portion.

The minimized value of the throat-pitch ratio “s/t” of the nozzle bladesis preferably a smallest value.

A geometrical discharge angle “α=sin⁻¹(s/t)”, which is calculated fromthe throat-pitch ratio “s/t” in the blade-root portion of the nozzleblades, is preferably set within a range of from at least 105% to up to115% of the geometrical discharge angle calculated from the minimumvalue of the throat-pitch ratio “s/t”.

The nozzle blades may have a cross section, which curves towards a fluidflowing side in the circumferential direction so that an extremelyprojecting portion exists in the blade-central portion in height.

The nozzle blades may incline or curve at a rear edge position thereoftowards either one of an upstream side opposing against the flow offluid and a downstream side following the flow of the fluid.

The nozzle blades may have a cross section so that a length of a chordof blade is maximized at the blade-tip portion and minimized at theblade-root portion.

The object of the present invention can be also achieved by providing,in another aspect, an axial turbine comprising: a plurality of turbinestages disposed in an axial direction of a turbine shaft, each of theplurality of turbine stages comprising a turbine nozzle unit havingnozzle blades, which are disposed in a row in a circumferentialdirection of an annular passage formed between an outer diaphragm ringand an inner diaphragm ring; and a turbine movable blade unit, which isdisposed on a downstream side of the turbine nozzle unit and has movableblades implanted in a row on the turbine shaft in a circumferentialdirection thereof, wherein the movable blades have a profile in which athroat-pitch ratio “s/t” is maximized at a blade-central portion inheight, wherein “s” being a shortest distance between a rear edge of amovable blade and a back side of another movable blade that is adjacentto the movable blade, and “t” being a pitch of the movable bladesdisposed in the row, minimized in a position between the blade-centralportion in height and a blade-root portion and increased from aminimized value to the blade-root portion.

In this aspect, the throat-pitch ratio “s/t”, which is increased fromthe minimized value to the blade-root portion, may be maximized at theblade-root portion.

In addition, a geometrical discharge angle “α=sin⁻¹(s/t)”, which iscalculated from the throat-pitch ratio “s/t” in the blade-root portionof the movable blades, may be set within a range of from at least 105%to up to 115% of the geometrical discharge angle calculated from theminimum value of the throat-pitch ratio “s/t”.

The movable blades may have a cross section, which curves towards afluid flowing side in the circumferential direction so that an extremelyprojecting portion exists in the blade-central portion in height.

The movable blades may incline or curve at a rear edge position thereoftowards either one of an upstream side opposing against the flow offluid and a downstream side following the flow of the fluid.

In addition, the object of the present invention can be also achieved byproviding, in a further aspect, an axial turbine comprising: a pluralityof turbine stages disposed in an axial direction of a turbine shaft,each of the plurality of turbine stages comprising a turbine nozzle unithaving nozzle blades, which are disposed in a row in a circumferentialdirection of an annular passage formed between an outer diaphragm ringand an inner diaphragm ring; and a turbine movable blade unit, which isdisposed on a downstream side of the turbine nozzle unit and has movableblades implanted in a row on the turbine shaft in a circumferentialdirection thereof, wherein the nozzle blades have a profile in which athroat-pitch ratio “s/t” is maximized at a blade-central portion inheight, wherein “s” being a shortest distance between a rear edge of anozzle blade and a back side of another nozzle blade that is adjacent tothe nozzle blade, and “t” being a pitch of the nozzle blades disposed inthe row, minimized in a position between the blade-central portion inheight and a blade-root portion, and increased from a minimized value tothe blade-root portion; and the movable blades have a profile in which athroat-pitch ratio “s/t” is maximized at a blade-central portion inheight, wherein “s” being a shortest distance between a rear edge of amovable blade and a back side of another movable blade that is adjacentto the movable blade, and “t” being a pitch of the movable bladesdisposed in the row, minimized in a position between the blade-centralportion in height and a blade-root portion and increased from aminimized value to the blade-root portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating a turbine nozzle unit appliedto an axial turbine according to the present invention, which is viewedfrom an outlet side of a main stream of a working fluid;

FIG. 2 is a perspective view illustrating a turbine movable blade unitapplied to an axial turbine according to the present invention, which isviewed from an outlet side of a main stream;

FIG. 3 is a cross-sectional view illustrating the turbine nozzle unitand the turbine movable blade unit applied to the axial turbineaccording to the present invention, in order to explain a flow passagethereof;

FIG. 4 shows throat-pitch ratio “s/t” distribution maps in comparisonbetween the prior art and the present invention, in which FIG. 4A is athroat-pitch ratio “s/t” distribution map of the turbine nozzle unit andFIG. 4B is a throat-pitch ratio “s/t” distribution map of the turbinemovable blade unit;

FIG. 5 shows loss distribution maps in which comparison in loss betweenthe prior art and the present invention is made, in which FIG. 5A is aloss distribution map of the turbine nozzle unit and FIG. 5B is a lossdistribution map of the turbine movable blade unit;

FIG. 6 is a distribution map of a loss variation amount showing arelationship between a geometrical discharge angle and the lossvariation amount in a blade-root portion of the turbine nozzle unit andthe turbine movable blade unit, which are applied to the axial turbineaccording to the present invention;

FIG. 7 illustrates blades, which are applied to the conventional axialturbine and viewed from the outlet side of the main stream, in whichFIG. 7A is a perspective view of the turbine nozzles and FIG. 7B is aperspective view of the turbine movable blades;

FIG. 8 is a conceptual view used for explaining the stream of the mainstream, which flows through the turbine nozzle unit and the turbineblade unit that are applied to the axial turbine according to thepresent invention;

FIG. 9 is a perspective view of another turbine nozzle unit applied tothe conventional axial turbine, viewed from the outlet side of the mainstream;

FIG. 10 is a conceptual view used for explaining the stream of the mainstream, which flows through the turbine nozzle unit applied to theconventional axial turbine;

FIG. 11 is a loss distribution map, which shows loss at a rear edge ofthe turbine nozzle blades applied to the conventional axial turbine; and

FIG. 12 is a conceptual view illustrating an example of stages of theaxial turbine provided with nozzle diaphragms.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereunder, embodiments of an axial turbine according to the presentinvention will be described with reference to the drawings. A steamturbine or a gas turbine is conceivable as the axial turbine describedbelow, and an example thereof is schematically shown in FIG. 12.

More specifically, FIG. 12 shows the stages of the axial turbine 100provided with nozzle diaphragms. Nozzle blades 104 are fixed to an outerdiaphragm ring 102 and an inner diaphragm ring 103, which are secured ina turbine casing 101, to form nozzle blade passages. A plurality ofturbine movable blades 106 is disposed on the downstream side of therespective blade passages. The movable blades 106 are implanted on theouter periphery of a rotor disc (wheel) 105 in a row at predeterminedintervals. A cover 107 is attached on the outer peripheral edges of themovable blades 106 in order to prevent leakage of a working fluid in themovable blades.

In FIG. 12, the working fluid, i.e., steam “S” flows from the right-handside (i.e., the upstream side) of the turbine in the figure towards theleft-hand side (i.e., the downstream side).

FIG. 1 is a perspective view of the turbine nozzle unit applied to theaxial turbine according to the present invention, which is viewed fromthe outlet side at the rear edge. In FIG. 1, a plurality of nozzleblades 1 is disposed at predetermined intervals in a row in acircumferential direction of an annular passage 4, which is formedbetween the outer diaphragm ring 2 and the inner diaphragm ring 3 andeach of the nozzle blades is connected, at a blade-tip portion andblade-root portion thereof, to the outer diaphragm ring 2 and the innerdiaphragm ring 3, respectively, so as to constitute a turbine nozzleunit.

FIG. 2 is a perspective view illustrating the movable blades 5, whichare disposed on the downstream side of the turbine nozzle unit relativeto the flow direction of the working fluid. Blade-tip portions aresupported by means of a shroud 7, and blade-implanted portions (i.e.,blade-root portions) are implanted into the rotor disc 6.

FIG. 3 shows a cross-section in a working fluid passage between thenozzle blades 1 and the movable blades 5. A throat-pitch ratio “s/t” isused as a parameter by which a flowing direction and an amount of theworking fluid from the outlet of the nozzle unit or the movable bladeunit is determined, wherein the throat “s” being the shortest distancebetween the rear edge of the nozzle blade 1 or the movable blade 5 and aback side of another nozzle blade 1 or another movable blade 5 that isadjacent to the former nozzle blade 1 or the former movable blade 5,i.e., the minimum passage width of the working fluid passage, and theannular pitch (i.e., the pitch of the movable blades disposed in therow) “t” being a number obtained by dividing the length in thecircumferential direction along a turbine shaft (not shown) by thenumber of nozzles or movable blades. A solid line in FIG. 4A shows thethroat-pitch ratio “s/t” of the nozzle blade 1, based on theabove-mentioned parameter, in the form of distribution in blade height,and a solid line in FIG. 4B shows the throat-pitch ratio “s/t” of themovable blade 5, based on the above-mentioned parameter, in the form ofdistribution in blade height.

In the axial turbine according to the present invention, thethroat-pitch ratio “s/t” of both of the turbine nozzle unit and theturbine movable blade unit is maximized at the blade-central portion inheight as shown in the solid lines in FIGS. 4A, 4B, in the same manneras the conventional unit as shown in the dotted lines in these figures.

In addition, in the axial turbine according to the present invention,the throat-pitch ratio “s/t” of both of the turbine nozzle unit and theturbine movable blade unit is minimized at a position between theblade-central portion and the blade-root portion, and the throat-pitchratio “s/t” at the blade-root portion is larger than that of theconventional unit as shown in the dotted lines.

In the axial turbine according to the present invention, the minimumvalue of the throat-pitch ratio “s/t” of the turbine nozzle unit is setas the smallest value in height of the blade, and the throat-pitch ratio“s/t” in the blade-root portion of the turbine movable blade unit is setas the largest value in height of the blade.

A blade profile in which the throat-pitch ratio “s/t” of both of theturbine nozzle unit and the turbine movable blade unit is maximized atthe blade-central portion in height, the throat-pitch ratio at theposition between the blade-central portion and the blade-root portion isminimized and the throat-pitch ratio is increased from this positiontowards the blade-root portion, can easily be realized, for example, byapplying a twist to the blade or changing the cross section of theblade.

The loss distribution of the turbine nozzle unit and the turbine movableblade unit is generally decreased at the blade-central portion inheight, on the one hand, and increased at the blade-root portion and theblade-tip portion, as shown in the dotted lines in FIGS. 5A, 5B. As aresult, in both of the conventional turbine nozzle unit and the turbinemovable blade unit, the main stream of the working fluid flows in alarger amount at the blade-central portion in height in which thesecondary flow loss (i.e., the secondary loss) of the working fluid issmall, on the one hand, and flows in a smaller amount at the blade-rootportion and the blade-tip portion, in which the secondary flow loss islarge, on the other hand.

In the embodiment of the present invention, the throat-pitch ratio “s/t”of both of the turbine nozzle unit and the turbine movable blade unit ismaximized at the blade-central portion in height as shown in the solidlines in FIGS. 4A, 4B, the throat-pitch ratio is minimized at theposition between the blade-central portion and the blade-root portionand the throat-pitch ratio “s/t” at the blade-root portion is increasedso that the main stream of the working fluid flows in a larger amount atthe blade-central portion in height where the secondary flow loss issmall, on the one hand, and flows in a smaller amount at the blade-rootportion and the blade-tip portion where the secondary flow loss islarge, on the other hand, thus making it possible to improve the turbinestage efficiency in comparison with the conventional unit. Especially,throat-pitch ratio “s/t” of both of the turbine nozzle unit and theturbine movable blade unit is minimized at the position between theblade-central portion in height and the blade-root portion and thethroat-pitch ratio is increased from this position towards theblade-root portion so as to reduce the loss such as the secondary flowloss, thus making it possible to further improve the turbine stageefficiency.

In addition, according to the embodiment of the present invention, thegeometrical discharge angle “α=sin⁻¹(s/t)” at the blade-root portion isincreased and the turning angle is decreased, thus making it possible toremarkably reduce the blade profile loss and the secondary flow loss incomparison with the conventional unit. FIG. 5A shows a loss distributionmap of the turbine nozzle unit and FIG. 5B is a loss distribution map ofthe turbine movable blade unit.

As shown in FIG. 6 based on analysis results, it is possible to reducethe loss by limiting the geometrical discharge angle “α=sin⁻¹(s/t)” atthe blade-root portion of the turbine nozzle unit and the turbinemovable blade unit within the range of 105%≦α≦115%, on the basis of theminimum value, more concretely, [(geometrical discharge angle at theblade-root portion α_(root)−the minimum value of geometrical dischargeangle α_(min))/(the minimum value of geometrical discharge angleα_(min))].

In the embodiment of the present invention, the throat-pitch ratio “s/t”distribution, which provides the profile, in which the throat-pitchratio “s/t” at the blade-central portion in height is minimized, thethroat-pitch ratio “s/t” at the position between the blade-centralportion in height and the blade-root portion is minimized and thethroat-pitch ratio “s/t” at the blade-root portion is increased, may beapplied to the so-called “compound lean type” turbine nozzle unit andturbine movable blade unit, as shown in FIGS. 7A, 7B. This can also beeasily realized by taking measures such as application of the twist tothe blades in cross section of the turbine nozzle unit and the turbinemovable blade unit.

In the turbine nozzle unit and the turbine movable blade unit, theblade-central portion in height in cross-section is shifted towards thecircumferential direction relative to the radial line “E”, and morespecifically, there exists an extremely projecting portion so as toproject at the blade-central portion in height from the nozzle blade 1or the movable blade 5 towards the back side “B” of the other nozzleblade 1 or the other movable blade 5, which is adjacent to the frontside “F” of the former blade 1 or 5, with the result that theabove-mentioned extremely projecting portion curves towards the flowingside of the main stream in the circumferential direction. A shiftingamount (i.e., an projecting amount) of this portion is determined basedon the magnitude of the secondary flow loss generated at the blade-rootportion and the blade-tip portion. With respect to the most suitablevalue for this shifting amount, an angle between the blade surface ofthe nozzle blade 1 or the movable blade 5 and the radial line “E” is 10°at the blade-root portion, on the one hand, and 5° at the blade-tipportion, on the other hand. The shifting amount (i.e., the projectingamount) exceeding the above-mentioned suitable value causes occurrenceof a drastic change in streamline, thus providing unfavorable effects.

Accordingly, a permissible range of the shifting amount (i.e., theprojecting amount) in cross-section of the blade is set as “10°±5°” at azone from the blade-root portion towards the blade-central portion inheight, on the one hand, and as “5°±5°” at a zone from the blade-tipportion towards the blade-central portion, on the other hand.

It is possible to cause, of the streams G₁, G₂, G₃ flowing between thenozzle blades 1 and then the movable blades 5, the stream G₁ to flowtowards the blade-root portion, on the one hand, and the stream G₃ toflow towards the blade-tip portion, on the other hand, as shown in FIG.8, thus leading to a low rate of occurrence of the secondary flow of theworking fluid, by applying the throat-pitch ratio “s/t” distribution,which provides the profile in which the throat-pitch ratio “s/t” at theblade-central portion in height is minimized, the throat-pitch ratio“s/t” at the position between the blade-central portion in height andthe blade-root portion is minimized and the throat-pitch ratio “s/t” atthe blade-root portion is increased in this manner, to the so-called“compound lean type” turbine nozzle unit and turbine movable blade unit,as shown in FIGS. 7A, 7B.

Alternatively, the throat-pitch ratio “s/t” distribution, which providesthe profile in which the throat-pitch ratio “s/t” at the blade-centralportion in height is minimized, the throat-pitch ratio “s/t” at theposition between the blade-central portion in height and the blade-rootportion is minimized and the throat-pitch ratio “s/t” at the blade-rootportion is increased, may be applied to the so-called “taper type”turbine nozzle unit and turbine movable blade unit.

In the so-called “taper type” turbine nozzle unit, the length of theblade chord “C” is gradually increased from the blade-root portiontowards the blade-tip portion on the observation based on the radialline “E”, as shown in FIG. 9, and the ratio of the blade chord “C” tothe annular pitch “t” is determined so as to reduce the blade profileloss in cross-section of the respective blade in the direction of theheight of the blade.

It is also possible to ensure a low rate of occurrence of the secondaryflow by applying the throat-pitch ratio “s/t” distribution, whichprovides the profile, in which the throat-pitch ratio “s/t” at theblade-central portion in height is minimized, the throat-pitch ratio“s/t” at the position between the blade-central portion in height andthe blade-root portion is minimized and the throat-pitch ratio “s/t” atthe blade-root portion is increased, to the so-called “taper type”turbine nozzle unit.

In the case where the throat-pitch ratio “s/t” distribution, whichprovides the profile, in which the throat-pitch ratio “s/t” at theblade-central portion in height is minimized, the throat-pitch ratio“s/t” at the position between the blade-central portion in height andthe blade-root portion is minimized and the throat-pitch ratio “s/t” atthe blade-root portion is increased, is applied to both of the turbinenozzle unit and the turbine movable blade unit, in the embodiment of thepresent invention, it is also possible to ensure a low rate ofoccurrence of the secondary flow by inclining or curving the rear edgeof each of the turbine nozzle blade and the turbine movable bladetowards the upstream side opposing against the flow of the main streamor the downstream side following the flow of the main stream.

It is therefore possible to remarkably reduce the loss of the turbinenozzle unit and the turbine movable blade unit and provide much power,to improve the efficiency of the turbine stage, when the throat-pitchratio “s/t” distribution, which provides the profile in which thethroat-pitch ratio “s/t” at the blade-central portion in height isminimized, the throat-pitch ratio “s/t” at the position between theblade-central portion in height and the blade-root portion is minimizedand the throat-pitch ratio “s/t” at the blade-root portion is increased,is applied, for example, to the so-called “compound lean type” turbinenozzle unit and turbine movable blade unit, or the “taper type” turbinenozzle unit and turbine movable blade unit, to constitute the turbinestage.

Industrial Applicability

According to the axial turbine according to the present invention, thethroat-pitch ratio “s/t” distribution, which provides the profile inwhich the throat-pitch ratio “s/t” at the blade-central portion inheight is minimized, the throat-pitch ratio “s/t” at the positionbetween the blade-central portion in height and the blade-root portionis minimized and the throat-pitch ratio “s/t” at the blade-root portionis increased, is applied to each of the turbine nozzle unit and theturbine movable blade unit to constitute the turbine stage. It istherefore possible to cause the main stream of the working fluid to flowin a larger amount at the blade-central portion in height so as toprovide much power, and increase the geometrical discharge angle“α=sin⁻¹(s/t)” at the blade-root portion so as to remarkably reduce theblade profile loss and the secondary flow loss of the working fluid.

According to the embodiment of the present invention, it is thereforepossible to improve remarkably the stage efficiency of the turbine stageto increase the power per the turbine stage.

1. An axial turbine comprising: a plurality of turbine stages disposedin an axial direction of a turbine shaft, each of the plurality ofturbine stages comprising a turbine nozzle unit having nozzle blades,which are disposed in a row in a circumferential direction of an annularpassage formed between an outer diaphragm ring and an inner diaphragmring; and a turbine movable blade unit, which is disposed on adownstream side of the turbine nozzle unit and has movable bladesimplanted in a row on the turbine shaft in a circumferential directionthereof, wherein said nozzle blades have a profile in which athroat-pitch ratio “s/t” is maximized at a blade-central portion inheight, wherein “s” being a shortest distance between a rear edge of anozzle blade and a back side of another nozzle blade that is adjacent tosaid nozzle blade, and “t” being a pitch of the nozzle blades disposedin the row, minimized in a position between the blade-central portion inheight and a blade-root portion and increased from a minimized value tosaid blade-root portion.
 2. An axial turbine according to claim 1,wherein said minimized value of the throat-pitch ratio “s/t” of thenozzle blades is a smallest value.
 3. An axial turbine according toclaim 1, wherein a geometrical discharge angle “α=sin⁻¹(s/t)”, which iscalculated from the throat-pitch ratio “s/t” in the blade-root portionof the nozzle blades, is set within a range of from at least 105% to upto 115% of the geometrical discharge angle calculated from the minimumvalue of the throat-pitch ratio “s/t”.
 4. An axial turbine according toclaim 1, wherein said nozzle blades have a cross section, which curvestoward a fluid flowing side in the circumferential direction so that anextremely projecting portion exists in the blade-central portion inheight.
 5. An axial turbine according to claim 1, wherein said nozzleblades incline or curve at a rear edge position thereof towards eitherone of an upstream side opposing against flow of fluid and a downstreamside following the flow of the fluid.
 6. An axial turbine according toclaim 1, wherein said nozzle blades have a cross section so that alength of a chord of blade is maximized at the blade-tip portion andminimized at the blade-root portion.
 7. An axial turbine comprising: aplurality of turbine stages disposed in an axial direction of a turbineshaft, each of the plurality of turbine stages comprising a turbinenozzle unit having nozzle blades, which are disposed in a row in acircumferential direction of an annular passage formed between an outerdiaphragm ring and an inner diaphragm ring; and a turbine movable bladeunit, which is disposed on a downstream side of the turbine nozzle unitand has movable blades implanted in a row on the turbine shaft in acircumferential direction thereof, wherein said movable blades have aprofile in which a throat-pitch ratio “s/t” is maximized at ablade-central portion in height, wherein “s” being a shortest distancebetween a rear edge of a movable blade and a back side of anothermovable blade that is adjacent to said movable blade, and “t” being apitch of the movable blades disposed in the row, minimized in a positionbetween the blade-central portion in height and a blade-root portion andincreased from a minimized value to said blade-root portion.
 8. An axialturbine according to claim 7, wherein said throat-pitch ratio “s/t”,which is increased from the minimized value to the blade-root portion,is maximized at the blade-root portion.
 9. An axial turbine according toclaim 7, wherein a geometrical discharge angle “α=sin⁻¹(s/t)”, which iscalculated from the throat-pitch ratio “s/t” in the blade-root portionof the movable blades, is set within a range of from at least 105% to upto 115% of the geometrical discharge angle calculated from the minimumvalue of the throat-pitch ratio “s/t”.
 10. An axial turbine according toclaim 7, wherein said movable blades have a cross section, which curvestowards a fluid flowing side in the circumferential direction so that anextremely projecting portion exists in the blade-central portion inheight.
 11. An axial turbine according to claim 7, wherein said movableblades incline or curve at a rear edge position thereof towards eitherone of an upstream side opposing against flow of fluid and a downstreamside following the flow of the fluid.
 12. An axial turbine comprising: aplurality of turbine stages disposed in an axial direction of a turbineshaft, each of the plurality of turbine stages comprising a turbinenozzle unit having nozzle blades, which are disposed in a row in acircumferential direction of an annular passage formed between an outerdiaphragm ring and an inner diaphragm ring; and a turbine movable bladeunit, which is disposed on a downstream side of the turbine nozzle unitand has movable blades implanted in a row on the turbine shaft in acircumferential direction thereof, wherein said nozzle blades have aprofile in which a throat-pitch ratio “s/t” is maximized at ablade-central portion in height, wherein “s” being a shortest distancebetween a rear edge of a nozzle blade and a back side of another nozzleblade that is adjacent to said nozzle blade, and “t” being a pitch ofthe nozzle blades disposed in the row, minimized in a position betweenthe blade-central portion in height and a blade-root portion, andincreased from a minimized value to said blade-root portion, and saidmovable blades have a profile in which a throat-pitch ratio “s/t” ismaximized at a blade-central portion in height, wherein “s” being ashortest distance between a rear edge of a movable blade and a back sideof another movable blade that is adjacent to said movable blade, and “t”being a pitch of the movable blades disposed in the row, minimized in aposition between the blade-central portion in height and a blade-rootportion and increased from a minimized value to said blade-root portion.